1. Field of the Invention
This invention relates to a network access protocol known as carrier sense multiple access with collision detection (CSMA/CD) and, more particularly, to a method for allowing isochronous data flow on such a network.
2. Description of the Related Art
The CSMA/CD protocol generally used in Ethernet LANs (local area networks), is defined in ANSI/IEEE standard 802.3, published by the Institute of Electrical and Electronics Engineers (hereinafter the "IEEE 802.3 standard"). Under the CSMA/CD rules for access to a network bus or cable, any node or station wishing to transmit must first listen to ensure that the channel is clear before beginning to transmit. All nodes on the network have equal priority of access and may begin transmitting as soon as the channel is clear and a required interpacket delay of 9.6 microseconds has elapsed. However, if a first node that has begun transmitting detects a collision with a transmission from another node, the first node continues transmitting for a short time to make sure that all nodes wishing to transmit will detect the collision (it is assumed that, while the attempts to transmit are nearly simultaneous, the first node is actually the first to begin). Collisions are detected by detecting a predetermined signal or voltage level on the bus. Every other node detecting the collision also continues to transmit for a short time. Then each node that has detected a collision terminates transmission of the packet or frame. The nodes involved in the collision then wait for a required interpacket delay of 9.6 microseconds and then select random and therefore usually different delay times, referred to as back-off times, before attempting to transmit their packets again.
The IEEE 802.3 standard defines a collision back-off procedure referred to as "truncated binary exponential back-off." When a transmission attempt has terminated due to a collision, it is retried by the transmitting node after a selected back-off time until either the transmission is successful or a maximum number of attempts have been made and all have been terminated due to collisions. The back-off time is selected by each node is an integral multiple of the "slot time" which is the maximum round trip propagation time for the network, i.e., the time required to propagate a data packet from one end of the network to another. The slot time is defined by the IEEE 802.3 standard as 51.2 microseconds. The number of slot times selected as the back-off time before the nth retransmission is chosen as a randomly distributed integer R in the range: 0.ltoreq.R.ltoreq.2.sup.k, where k=min (n, 10).
While generally adequate for transmitting packetized burst-type data such as e-mail or word processing documents, a CSMA/CD network according to the IEEE 802.3 protocol makes no provision for traffic priority. Thus, real-time or isochronous (i.e., higher priority) traffic is put at risk of being blocked if a node is currently transmitting lower priority data. High priority isochronous data such as voice or video requires a guaranteed bandwidth and tightly bounded delivery delays. Accordingly, the delay required to complete the transmission of lower priority data can adversely affect the higher priority data.
One approach to providing for isochronous data transmission on a local area network is isochronous Ethernet ("isoEthernet") or IEEE standard 802.9a. IsoEthernet is a hybrid network that combines standard 10 megabit per second Ethernet with 6.144 megabits per second of isochronous bandwidth for a total of 16 megabits per second available to any user. The isochronous portion is further divided into 96 separate 64 kbps ISDN bearer or B channels. While providing backward compatibility and the ability to be introduced piecemeal, isochronous Ethernet requires channels separate from the existing CSMA/CD data path in order to provide for isochronous data flow. This results in a relatively higher and undesirable level of complexity.
Accordingly, it is desirable to provide a system and method for reducing the latency of high priority data on an existing CSMA/CD network path. There is a still further need for providing for interrupting low priority transmission so that a higher priority transmission may occur on the same channel. Finally, there is a need for resuming transmission of the lower priority transmission once the higher priority transmission has been completed.